How do search engines work?

The term "search engine" is often used generically to describe both true search engines and directories. They are not the same. The difference is in how listings are compiled.

Search Engines Vs. Directories
Search Engines: Search engines, such as Google, create their listings automatically. Search engines crawl the web, then people search through what they have found.

If you change your web pages, search engines eventually find these changes, and that can affect how you are listed. Page titles, body text and other elements all play a role.

Directories: A directory such as Yahoo depends on humans for its listings. You submit a short description to the directory of your entire site (or editors write one for sites they review). A search looks for matches only in the descriptions submitted.

Changing your web pages has no effect on your listing. Things that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed than a poor site.

Hybrid Search Engines: Some search engines maintain an associated directory. Being included in a search engine's directory is usually a combination of luck and quality. Sometimes you can "submit" your site for review, but there is no guarantee that it will be included.

The Parts Of A Search Engine
Search engines have three major elements. First is the spider, also called the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what is meant by a site being "spidered" or "crawled." The spider returns to the site on a regular basis, such as every month or two, to look for changes.

Everything the spider finds goes into the second part of a search engine, the index. The index, sometimes called the catalog, is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated with the new information.

Sometimes it can take a while for new pages or changes that the spider finds to be added to the index. Thus, a web page may have been "spidered" but not yet "indexed." Until it is indexed -- added to the index -- it is not available to those searching within the search engine.

Search engine software is the third part of a search engine. This is the program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in order of what it believes is most relevant.

Major Search Engines: The Same, But Different
All search engines have the basic parts described above, but there are differences in how these parts are tuned. That is why the same search on different search engines often produces different results.